Comparative Inclusion Analysis in Nitinol Fine Wire

The influence of inclusions in initiating fatigue fracture in superelastic nitinol, utilized in the manufacture of self-expanding medical devices, has long been understood and many efforts have tried to predict the fatigue performance of a material based on micro-cleanliness. ASTM F2063 specifies that the micro-cleanliness (non-metallic inclusions and associated porosity/voids) is evaluated at an intermediate processing stage, which are many times larger than final material used in the manufacturing of typical self-expanding stents. There is uncertainty as to a relationship between the upstream product analysis, per ASTM F2063, and the final fine wire inclusion dimensions. In this investigation, two materials with different rotating bending fatigue performance, sourced from different melters, were evaluated at the fine wire stage for inclusion size/distribution with three techniques including: metallography, fractography, and electrochemical matrix dissolution with inclusion recovery. This work focuses on comparing the inclusions measured through the different techniques and highlighting the advantages and limitations of the various methods. Additionally, the work looks at the inclusion size measurements from these techniques holistically to understand which of these is a better indicator of the material’s fatigue response.